Power steering for motor vehicles

ABSTRACT

A power steering system for automobiles has a mechanical drive connection between a steering device and a steering gear arrangement for steering of vehicle wheels. The drive connection can be separated by opening a connection. In that event, the steering device is connected to the steering gear arrangement only indirectly, in that only nominal values for the steering angle of the wheels steering the vehicle are specified as a function of the adjusting travel of and/or the adjusting speed of the steering device, and the servo motor is actuated by a separate control valve arrangement.

This application claims the priority of German application 195 40 956.6and PCT/EP96/04734, filed Nov. 3, 1995 and Oct. 31, 1996, respectively,the disclosure of which are expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to power steering for automobiles with amechanical drive connection between a steering device, particularly asteering wheel, and a steering gear arrangement for steering of vehiclewheels, a servo motor coupled with the steering gear arrangement and aconnection for separating the drive connection. The servo motor iscontrollable over a control valve which, in turn, can be controlled by acontrol system as a function of a difference between a first value,correlated with the steering angle of the vehicle wheels, and at leastone second value, correlated with the adjusting travel of the steeringdevice. A servo valve additionally is connected with the driveconnection and analogous thereto, can be controlled by the forces ormoments transferred by the drive connection, over which servo valve theservo motor can be connected in a controllable manner with delivery andlow-pressure connections. Thereby, the servo motor produces a servopower reducing the actuation force necessary at the steering device andthe servo motor being connected through a reversing valve arrangementeither to the motor connections of the control valve or to the motorconnections of the servo valve.

Such power steering arrangements are known from JP-A-40 19271. Steeringarrangements of this type have two operating states. In one operatingstate, a mechanical forced connection is provided between the steeringdevice and the steered vehicle wheels whereby, as a function of theforces transferred between the steered vehicle wheels and the steeringdevice, a servo valve is controlled which, in turn, actuates a hydraulicservo motor. In this operating state, the steering therefore works likeconventional hydraulic power steering. In the other operating state, thesteering device is disconnected from the steered vehicle wheels, and theservo motor is controlled by a control system as a function of adifference between a first value correlated with the steering angle ofthe steered wheels of the vehicle and a second value correlated at leastwith the adjusting travel of the steering device. In this state, thesteering therefore works according to the "steer by wire" concept.

To switch between the two operating states at need, a reversing valvearrangement connects the servo motor either with the motor connectionsof the servo valve or with the motor connections of the servo valveactuated by the control system. Therefore, in the event that the controlsystem malfunctions, it is possible to switch over to the operatingmethod of the conventional power steering system.

In JP-A-40 19271, the valve arrangement, which is provided for theabove-cited switching, normally is in the position provided for the"steer by wire" operating state. The reversing valve arrangement isswitched hydraulically from this normal position when the servo valve isactuated with the mechanical forced connection switched on between thesteering device and the steered wheels of the vehicle. The servo motoris built in a special manner for this purpose. There are objections tothis concept for safety reasons.

A vehicle steering system is described in WO 90/12723 in which themechanical forced connection, provided for emergency purposes between asteering wheel and the wheels steering the vehicle, is normallyseparated. In this normal state, a first hydraulic adjusting unit,serving to adjust the steering of the wheels steering the vehicle and asecond hydraulic adjusting unit, serving to produce a manual forceperceptible at the steering wheel, are actuated. Both adjusting unitsare controlled by electronics. These control electronics, on one hand,actuate the first hydraulic adjusting unit assigned to the steeringwheels of the vehicle in accordance with signals from a nominal valuetransmitter actuated by the steering wheel for the desired angle ofsteering. On the other hand, the second hydraulic adjusting unit,assigned to the steering wheel and serving for production of manualpower, is actuated by the control electronics, whereby various types ofparameters can be taken into consideration. The control electronicscreate the possibility of adjusting the ratio between the change insteering angle of the wheels steering the vehicle and the change inangle of rotation of the steering wheel to a value desired for goodhandling of the vehicle, without having to demand an excessiveexpenditure of force on the part of the driver, because the driver needonly overcome the resistance of the second adjusting unit serving toadjust the manual force. The actual forces necessary for the steeringadjustment of the wheels steering the vehicle are applied by the firstadjusting unit. In the final analysis, an easy, but nevertheless directsteering is thus created, in which a relatively small shift in rotationof the steering wheel leads to a clear change in angle of steering ofthe wheels steering the vehicle. only in emergency cases is a mechanicalforced connection switched in between the steering wheel and the wheelssteering the vehicle. Comparatively indirect steering, i.e., arelatively large shift in rotation of the steering wheel leads to only arelatively small change in the angle of steering of the steered wheelsof the vehicle, must be accepted in this system for limiting the manualforce necessary.

A conventional power steering is described in DE 44 09 928 A1, in whichthe steering wheel is constantly connected mechanically with the steeredwheels of the vehicle. A pump serving as a source of hydraulic pressureis combined with a hydraulic pressure accumulator which can be chargedby the pump as required or maintained at a specified boost pressure. Thepressure accumulator is connected over a pressure line controlled by acheck valve to a pressure connection of the servo valve. This pressureconnection is also connected with the delivery side of the pump over acontrol line, in which a throttle is located. In phases, in which thereis no need for servo support, the servo valve is in an open centralposition, in which the above-cited pressure connection is connected withboth the servo motor connections and a low-pressure connection. Thereby,the servo valve is connected with a relatively low-pressure hydraulicreservoir. The hydraulic stream flowing over the above-mentioned controlline from the pump to the servo valve is fed to the reservoir over theservo valve, as long as the servo valve is in the state with its opencenter. If, now, during a steering maneuver, the servo valve isincreasingly shifted, increasing throttle resistance arises between thepressure connection and the low-pressure connection of the servo valvewith the consequence that the hydraulic pressure in the control lineincreases sharply. This pressure is transmitted to a control input ofthe control valve between the pressure accumulator and the servo valvewith the consequence that the control valve is now opened and thepressure line released, that is, the pressure connection of the servovalve is connected with the pressure accumulator. Thus, a sufficientlylarge hydraulic stream, supplied under high pressure from the pressureaccumulator and/or pump, is available for producing a larger servoforce.

An object of the present invention is to ensure a high degree of safetyfor a power steering system of the above-described type.

This object has been accomplished owing to the fact that the servo motorcan be connected over normally-open reversing valves with the motorconnections of the servo valve and over normally closed reversing valveswith the motor connections of the control valve and these reversingvalves can be controlled by the control system into their other positiononly if the control system is working properly.

In the present invention, therefore, the reversing valve arrangementnormally is in the position necessary in the event of a malfunctioningof the control system, i.e., when the steering is supposed to work as aconventional hydraulic power steering system. Only when the controlsystem is operating perfectly can the reversing valve arrangement assumethe other position for the "steer by wire" operating mode, provisionhaving been made for the control system to carry out this valvereversal.

The present inventive system excels due to the greatest degree ofsafety, since it is possible, without anything further, for theconnection to switch without delay into its closed state and thusproduce the mechanical connection between steering device and thesteering wheels of the vehicle as soon as errors are detected in thecontrol system which, in a manner known in principle, can constantlycheck itself for error-free function.

With the connection open, in principle, certain conversion relationshipscan be realized between the steering device and the steering wheels ofthe vehicle. In addition, when the nominal values for the steering angleof the steering wheels of the vehicle are specified, disruptive forcesaffecting the vehicle, such as crosswinds or other parameters, such as,for example, yaw rate, can be taken into consideration.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawingswherein:

The sole figure is a wiring diagram-like schematic representation of thepower steering of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

A steering wheel 1 is, in a manner known in principle, connected by asteering column 2 with a pinion 3 which meshes with a steering rack 4connected, by a steering tie rod 5 with the steering wheels 6 of thevehicle so that vehicle steering wheels 6 are steered in the appropriatedirection when the steering wheel 1 is turned to the left or the right.

Between the pinion 3 and the steering column 2, a rotary-type servovalve 7 is arranged and has two control parts that are connected to eachother in a torsionally-elastic manner by a torsion bar (not shown), ofwhich one part is connected, non-rotationally, to the pinion 3 and onepart is connected, non-rotationally, to the steering column 2. As afunction of the torque transferred between the pinion 3 and the steeringcolumn 2, the two control parts of the servo valve 7 are rotated more orless relative to each other to control servo valve 7 in the mannerexplained below.

An input line 8 connects the servo valve 7 with the delivery side ofhydraulic pump 9 which is connected at the inlet side to the hydraulicreservoir 10, with which servo valve 7 is connected over the outlet line11. In addition, the servo valve 7 is connected to two motor lines 12,13 that each communicate with a chamber of double-acting piston-cylinderunit 14 serving as servo motor in the illustrated embodiment, the pistonof which is disposed on a piston rod continuing the steering rack 4. Thesystem thus far described is a conventional power steering system.

In the present invention, a connection 15 is disposed in the steeringcolumn 2 for separating the mechanical drive connection between thesteering wheel 1 and the steered wheels 6. On the steering wheel side ofthe connection 15, a manual force adjuster 16 is disposed on thesteering column 2 and is formed, for example, as a non-self-lockingelectromotor. In this way, a controllable actuation resistance can becreated at the steering wheel 1, particularly when the connection 15 isopen.

Normally-open check valves 17, 18 and 19 are located in the inlet line 8as well as in the motor lines 12, 13. These check valves 17, 18 and 19can each be switched into a closed position by assigned electromagnetsagainst the force of restoring springs. As soon as the electric currentsupplied to the electromagnets is switched off, the valves 17, 18 and 19return to the open position shown.

From the delivery side of the hydraulic pump 9, an additional line 20leads, over nonreturn valve 21 which prevents backflow into pump 9, to apressure accumulator 22 as well as to a connection to a slide valvearrangement 23. An additional connection from this slide valvearrangement 23 is connected to the hydraulic reservoir 10 over a returnflow line 24. Two additional connections from the slide valvearrangement 23 are connected with the motor lines 12, 13 over lines 25,26 which are controlled by check valves 27, 28, between the check valves18 and 19 and piston-cylinder unit 14.

The slide valve arrangement 23 is forced by a spring arrangement intothe illustrated middle position, in which lines 25, 26 are shut off fromthe other two connections of the slide valve arrangement 23.Electromagnets shift the slide valve arrangement 23 in two opposingdirections in such a manner that a more or less sharply throttled orunthrottled passage is opened either between the pressure accumulator 22and the line 25, and the line 26 and the line 24, or between thepressure accumulator 22 and the line 25, and the line 25 and the line24.

The check valves 27, 28 can be switched from their illustratednormally-closed position into their open position against the force ofrestoring springs also by electromagnets. When the electric currentsupplied to the electromagnets is switched off, the check valves 27, 28return to the illustrated closed position.

The valves 17 through 19 and 23 as well as 27, 28 are actuated bycontrol electronics 29 that also control the manual force adjuster 16,and can control the pump 9. On the inlet side, the control electronics29 are connected to a sensor 30 for the rotational position of thesteering wheel 1 as well as a sensor 31 whose signals reproduce thesteering angle of the steered vehicle wheels 6. Furthermore, the controlelectronics 29 are connected on the inlet side with a pressure sensor 32that records the pressure in pressure accumulator 22.

In describing the operation of the system, it is assumed that theconnection 15 is closed and all the check valves 15 through 18 and 27,28 are in the illustrated positions. In this event, the steeringoperates like conventional power steering. The slide parts of the servovalve 7 are shifted relative to one another to an extent which dependson the torque transferred over the steering column 2. This results inthe fact that, when the hydraulic pump 9 is operating between the motorlines 12, 13, a more or less large pressure difference arises in one orthe other direction, depending on the direction of the torque, and thatthe piston-cylinder unit 14 produces a correspondingly large servo forcein one or the other direction, so that the manual force to be applied atsteering wheel 1 during the respective steering maneuver is clearlyreduced.

If necessary, the control electronics 29 can operate the manual forceadjuster 16 as a function of specified parameters, such as the drivingspeed, so that the resistance perceptible at the steering wheel 1 ischanged in a specifiable manner.

An essential characteristic feature of the present invention resides inthe fact that with specifiable driving conditions that can be determinedby the sensors (not shown) communicating with the control electronics29, the connection 15 is opened. The control electronics 29 then switchall of the check valves 17 through 19 and 27, 28 into their other, notshown position. This results in the fact that the servo valve 7 becomesinoperative and in particular, can no longer communicate with thepiston-cylinder unit 14. Rather, the piston-cylinder unit 14 is nowconnected with the slide valve arrangement 23 which is actuated by thecontrol electronics 29. In this manner, the slide valve arrangement 23is actuated so that, in the final analysis, the actual value of thesteering angle of the wheels 6 steering the vehicle, determined by thesensor 30, corresponds to a nominal value specified by the angle ofrotation of the steering wheel 1 determined by the sensor 30. Here,practically any desired conversion ratios can be created between theangle of rotation of the steering wheel 1 and the steering angle of thesteered wheels 6.

In this manner, the conversion ratio can be changed particularly as afunction of the speed of rotation of the steering wheel 1 that can alsobe determined by the sensor 30.

With this method of operation, the pressure difference can be varied atthe piston-cylinder unit 14 in one or the other direction by theseparate slide valve device 23, depending on the deviation of the actualfrom the nominal value, so that the piston-cylinder unit 14 brings aboutthe desired steering motion of the wheels 6.

As soon as the desired steering angle is set, the slide valvearrangement 23 is returned to the illustrated closed position shown, inwhich the piston-cylinder unit 14 is blocked hydraulically. Since thepiston-cylinder unit 14 is also now separated from the hydraulic pump 9,the pressure accumulator 22 is recharged until the pressure sensor 32reports that the upper pressure threshold has been reached. The pump 9can then be shut down. As soon as the control valve arrangement 23subsequently is triggered out of the closed center position because anew steering maneuver is to occur, hydraulic pressure is supplied to thepiston-cylinder unit 14 by the pressure accumulator 22. In addition, thehydraulic pump 9 is switched on again as soon as the pressure fallsbelow the lower threshold value at the pressure sensor 32.

Optionally, the hydraulic pump 9 can also run continuously. Here thecirculated, unnecessary oil can flow through the valve 17 to control thevalve 7 and through its open center over the flow return line 11 intothe tank. The valves 18, 19 are closed here, while the control valve 23is active.

With the connection 15 open, the control electronics 29 also constantlycontrol the manual force adjuster 16 so that an adjusting resistance isperceptible at the steering wheel 1. Conditions, similar to thosecreated by a mechanical connection between steering wheel 1 and wheels 6can be simulated at the steering wheel 1.

If the control electronics 29 are provided with appropriate sensors (notshown), they can also execute special steering maneuvers in order, forexample, to stabilize the vehicle after or during a crosswind gust or tohalt a skidding motion.

Control electronics 29 constantly monitors itself for correct operation.As soon as an error has been detected, the current supplied to theelectromagnets of the reversing valves 17 through 18 and 27, 28 isimmediately shut off so that these valves switch to the illustratedpositions. At the same time, the actuator for the connection 15,generally an electric one, is cut off from the power supply so that theconnection 15 is switched to its closed position by a closing spring orthe like. As a result, this creates a return to conventional powersteering with the mechanical connection between the steering wheel 1 andthe wheels 6 steering the vehicle.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. Power steering system for a vehicle with amechanical drive connection between a steering device and a steeringgear arrangement for steering wheels of the vehicle, comprising a servomotor connected to the steering gear arrangement, connection forseparating the drive connection, the servo motor being controllable withthe connection in an open position, over a control valve configured tobe controlled by a control system as a function of a difference betweena first value correlated with the steering angle of the vehicle wheelsand a second value correlated at least with the adjusting travel of thesteering device, and a servo valve configured to be controlledanalogously to forces or moments transferred by the drive connection andcouplable with the drive connection for controllably connecting theservo motor with pressure and low-pressure connections such that theservo motor produces servo power reducing the actuating force necessaryat the steering device, the servo motor being connected by a reversingvalve arrangement with one of motor connections of the control valve andmotor connections of the servo valve, wherein the servo motor isconfigured to be connected over normally opened reversing valves withthe motor connections of the servo valve and over normally closedreversing valves with the motor connections of the control valve, thereversing valve arrangement configured to be controlled into arespective other position only in a proper working condition of acontrol system.
 2. The power steering system of claim 1, wherein theservo valve has an open center.
 3. The power steering system of claim 1,wherein the separate control valve has a closed center.
 4. The powersteering system of claim 1, wherein a pressure accumulator isoperatively arranged at a line connecting a servo pump on delivery sidethereof with the separate control valve, and a nonreturn valve isarranged to prevent return flow in the direction of the pump.
 5. Thepower steering system of claim 1, wherein the servo motor is connectableover normally-open reversing valves with motor connections of the servovalve and over normally-closed reversing valves with motor connectionsof the separate control valve.
 6. The power steering system of claim 1,wherein the servo motor is connectable over normally-open reversingvalves with motor connections of the servo valve and overnormally-closed reversing valves with motor connections of the separatecontrol valve.
 7. The power steering system of claim 3, wherein theservo valve has an open center.
 8. The power steering system of claim 4,wherein the servo valve has an open center.
 9. The power steering systemof claim 4, wherein the separate control valve has a closed center. 10.The power steering system of claim 5, wherein a pressure accumulator isoperatively arranged at a line connecting a servo pump on delivery sidethereof with the separate control valve, and a nonreturn valve isarranged to prevent return flow in the direction of the pump.
 11. Thepower steering system of claim 8, wherein the separate control valve hasa closed center.
 12. The power steering system of claim 11, wherein apressure accumulator is operatively arranged at a line connecting aservo pump on delivery side thereof with the separate control valve, anda nonreturn valve is arranged to prevent return flow in the direction ofthe pump.